Which way does the coolant flow?

Centrifugal pumps push. They will not pull water very high, but they will push it up a well, pretty good.
If your heater core has different sizes of pipes, they are keyed likewise to the engine fittings. I think pre about 72, that was the case.If you hook em backwards, they can trap an air-bubble and it will be noisy for a long time, until the air bubble works itself out.